1. Field of the Invention
The present invention is directed to a telerecording system for producing x-ray images having an x-ray image intensifier video chain which includes a semiconductor image converter.
2. Description of the Prior Art
Due to the exponential absorption which occurs in x-ray images produced by x-ray diagnostics installations of the above type, high differences in intensity are always present, i.e. the dynamics (contrast ranges) of the x-ray image are extremely high.
In semiconductor image converters, particularly CCD sensors, the amplifier noise is uniformly superimposed on all signal values, thus it has the greatest disturbing effect given low signal values, i.e. in the dark portions of the image. In order to boost these dark portions of the image, the optical modulation of the semiconductor image converter can be raised, however, the bright sections of the image would then move into the overmodulation or blooming region because of the linear characteristic of the semiconductor image converter, so that important information would be obscured. Neighboring dark regions of the image, moreover, also become deteriorated due to blooming in the x-ray image.
The limitation of the range of modulation given CCD converters is established by the barrier effect. An effective barrier for the charge carriers relative to the neighboring elements is no longer present when a defined charge amount per image element in the CCD converter is exceeded and the charge therefore flows over the previously-existing (blooming). This blooming can be so pronounced that large sections of the image become unusable dependent on the CCD principle employed. Modern CCD sensors therefore include so-called anti-blooming devices. These are composed, for example, of diverting or bypass diodes respectively allocated to every picture element which divert excessive charges before the blooming effect occurs. The common terminal of all anti-blooming diodes is connected, for example to the substrate and is available as a separate electrical terminal. An electrical "blooming" is thereby in fact prevented; the image locations, however, continue to be too bright and contain no information. Both a mechanical as well as an electrical darkening lead to a poorer signal-to-noise ratio. Even if control of the semiconductor image converter sensitivity with a shutter function is undertaken, a loss of x-ray quanta still occurs since an integration of charges on the basis of acquired light only ensues in a part of the exposure time.